Oxana Eschenko

The scientific interest of my research group is to better understand the neurophysiological mechanisms of noradrenergic (NA) neuromodulation in the brain. The NA system mediates many cognitive processes such as attention, perception, learning and memory. Dysfunction in noradrenergic system often leads to various psychiatric disorders.

The core of the NA system is a small brainstem nucleus Locus Coeruleus (LC). The LC-NA neurons project extensively throughout the brain, innervating the spinal cord, the brain stem, hypothalamus, cerebellum, thalamic relay nuclei, amygdala, basal telencephalon, as well as the cortex. As opposed to direct synaptic neurotransmission (wired transmission), the axons of NA neurons commonly have characteristic varicosities by means of which NA is secreted and diffused through large volumes of tissue (volume transmission), thereby affecting simultaneously multiple and diverse population of neurons. The mechanisms underlying the specificity of neuromodulatory effects at different projection targets are poorly understood. A major advance in understanding the neurophysiological mechanisms of NA neuromodulation in the brain may be achieved by monitoring neuromodulatory effects at multiple brain regions simultaneously and at different scales.

The specific aims are the following: 1) to describe the effective (functional) connectivity of the LC in the rat brain; 2) to characterize temporal interactions between LC activity and its cortical and subcortical targets during spontaneous and sensory-evoked activity in anesthetized and behaving rats; and 3) to study how the modulation of LC activity affects cortical state, sensory responses, and sensory-guided behavior.

Project 1. Mapping of the functional connectivity of the LC in the rat brain.

We compared the labeling produced by a classical anatomical tracer (fluorescent dextran) and by MRI-visible tracer (Mn2+) injected simultaneously in LC. The major cortical and subcortical targets of LC projections including predominantly ipsilateral primary motor (M1) and somatosensory (S1) cortices, hippocampus and amygdala were detected using manganese-enhanced MRI (MEMRI). MEMRI method consistently failed to reliably label several minor but also major targets of LC, notably the thalamus (Eschenko et al., 2011). The lack of Mn2+ labeling in thalamus possibly reflected a weaker functional connectivity within coeruleothalamic projections that could not be predicted by anatomical tracing. These results will be complemented by mapping of the functional connectivity of the LC projections using combined microstimulation and fMRI.

We compared the effects of systemic (i.p.) or local (into LC) application of clonidine, an alpha2-receptor agonist, which is known to inhibit LC-NA neurons, on sensory responses in S1 and PFC, the two cortical targets of LC. Local application of clonidine resulted in complete cessation of both spontaneous and evoked activity of LC neurons to mild foot shocks (FS). Absence of LC signaling did not affect S1 responses, while both increased and decreased responses were observed in PFC (Fig.1). Systemic clonidine produced a transient decrease of LC spontaneous activity, while LC evoked responses were preserved. This manipulation decreased signal-to-noise ratio (SNR) in S1 neurons, while sensory signaling in PFC was, overall, increased. We now extend this project to recordings in VTA.

This project is collaboration with Dr. S. Sara and a part of the PhD study of S. van Keulen.

Project 3. Noradrenergic modulation of the cortical state and state-dependent sensory coding.

The neural responses to sensory stimulation are more robust when cortical activity is decorrelated (or desynchronized state). Moreover, the sensory responses markedly differ when stimulus occurred in the depolarized (Down) or hyperpolarized (Up) state. Inhibition of LC activity (e.g. by clonidine) leads to more synchronized activity in cortex. The electrical microstimulation of LC (e.g. trains of pulses at 50Hz for 500ms) applied during synchronized state produces a transient (~1-2s) desynchronization. The sensory responses resembled such during Up state if LC stimulation preceded the sensory stimulus.

This project is collaboration with Dr. S. Panzeri and Dr. C. Magri and a part of master thesis of R. Neves.

Project 4. Investigation of the effects of electrical microstimulation of the Locus Coeruleus in anesthetized and behaving rats.

We performed recording/stimulation unsing the same electrode tip placed in the LC. Electrical stimulation produced a sustained inhibition(40-120ms) of LC neurons at the stimulation site. There was no effect of pulse duration (range: 0.1-0.5ms) or current intensity (range: 0.01-0.2mA) on LC inhibition at the stimulation site. The linear relations between both factors and the duration of LC inhibitions were onserved at longer distances from the stimulation site. Neural responses in the contralateral LC showed overall a shorter inhibition (65±6ms). Trains of pulses (>200ms at 20-50Hz) delivered to the LC resulted in a transient desynchronization in mPFC.

This project is collaboration with Dr. A. Marzo.

Our results demonstrate that:

1) functional connectivity of LC with its projection targets could not be predicted from anatomical connectivity and may be context- or state dependent;

2) blocking the LC sensory-evoked discharge differentially affected signal processing in S1 and PFC as the opposite effects (decrease and increase in SNR) were observed with overall stronger modulation in PFC;

3) LC-NA system is involved in regulation of cortical state and therefore affect state-dependent sensory processing.

4) Application of the electrical current to LC, as low as 0.01mA, may mimic a characteristic response of the LC-NE neurons to salient stimuli (a brief excitation followed by a prolonged inhibition), however only a relatively strong LC stimulation (trains of pulses) affect neural activity in the distal cortical targets of LC, e.g. mPFC.

Eschenko O (April-29-2015) Invited Lecture: The role of Locus Coeruleus for sensory processing within mesocortical dopaminergic pathway , SFB 874 / IGSN Conference: Cortical and Subcortical Representation of Sensory and Cognitive Memory , Bochum, Germany. Salient events evoke burst-like responses of noradrenergic (NE) neurons of the Locus Coeruleus (LC) and dopaminergic (DA) neurons of the ventral tegmenta l area (VTA). The associated NE and DA release modulates information processing in the projection targets of LC and VTA. In the rat, terminal fields of both LC-NE and VTA-DA neurons converge in the medial prefrontal cortex (mPFC), a cortical area controlling many cognitive capacities. We investigated the role of LC phasic activation for sensory responses in VTA and mPFC. Under urhetaine anestesia, noxious stimulatio
n (foot shock, FS) produces a robust short-latency (~20 ms) excitation of LC-NE neurons. In VTA and mPFC, the firing rate modulation induced by FS was present in ~30% of neurons. We classified FS-induced responses of VTA neurons according to latency (early: ~40 ms or late: ~150 ms) and duration (phasic: < 300 ms or sustained: > 300 ms). Similarly, the mPFC single-unit responses differed by latency and/or duration. Supression of LC ongoing and FS-evoked activity by iontophoretic injection of clonidine,
an alpha2-adrenergic receptor agonist, reduced responsiveness in both VTA and mPFC. Population of
initially ‘non-responsive’ mPFC neurons showed
‘gating-effect’. Spontaneous discharge of substantial proportion of VTA and mPFC neurons was bidirectionally modulated. These results suggest that depending on the motivational valence of a salient event, LC phasic activation and associated NE release may selectively enhance or supress signalling within different and, possibly competing mesolimbic and mesocortical pathways. The behavioral data supporting this hypothesis will be presented. CiteID: Eschenko2015

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Eschenko O (July-6-2014) Invited Lecture: The role of Locus Coeruleus for sensory processing within the mesocortical dopaminergic pathway, 9th FENS Forum of Neuroscience, Milano, Italy9 ( R10101) . Salient events evoke burst-like responses of noradrenergic (NE) neurons of the Locus Coeruleus (LC) and dopaminergic (DA) neurons of the ventral tegmental area (VTA). The associated NE and DA release modulates signal processing in the projection targets of LC and VTA, which is beneficial for selection of adaptive behavioral response. In the rat, terminal fields of both LC-NE and VTA-DA neurons converge in the medial prefrontal cortex (mPFC), a cortical area controlling many cognitive capacities. We investigated the role of LC phasic activation for sensory representations in two LC targets by simultaneous electrophysiological recording in LC, VTA and mPFC and pharmacological manipulation of LC activity. Under urhetaine anestesia, noxious stimulation (foot shock, FS) produces a robust short-latency (~20 ms) excitation/inhibition response of LC-NE neurons. Populations of VTA and mPFC neurons also exhibit phasic excitatory and inhibitory responses, yet with longer latencies (~100 ms). Supression of LC spontaneous and evoked activity by iontophoretic injection of clonidine, an alpha2-adrenergic receptor agonist, disinibited a substantial proportion of VTA-DA and mPFC pyramidal neurons regardless of their FS-responsiveness. Furthermore, LC inhibition bidirectionally modulated the VTA-DA and mPFC resposes to noxious stimulation. The ongoing and evoked activity of VTA non-DA neurons was unaffected. These results suggest that depending on the motivational valence of a salient event, the LC-NE system may selectively enhance or supress signalling within different and, possibly, competing mesolimbic and mesocortical pathways. This hypothesis is being currently tested in behaving animals engaiged in a sensory cue-guided reward-motivated operant task. CiteID: Eschenko2014

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Eschenko O (March-17-2014) Invited Lecture: Ripple-triggered stimulation of Locus Coeruleus during post-learning sleep impairs memory consolidation, 34th European Winter Conference on Brain Research and European Brain and Behaviour Society (EWCBR/EBBS 2014), Brides-les-Bains, France. Hippocampal ripples, brief high-frequency (150-200Hz) oscillations occurring during quiet wakefulness or slow wave sleep (SWS), represent simultaneous discharge of a large neuronal population that is synchronized across the entire hippocampus. Learning experience increases frequency of ripple occurrence, which is predictive of memory recall, while ripple suppression impairs hippocampal-dependent learning. Experience-induced replay of neuronal ensembles occurs predominantly during ripples. These observations support the idea that ripples provide a neurophysiological substrate for ‘off-line’ memory consolidation by
facilitating synaptic plasticity within the learning-associated neuronal network. We hypothesized that noradrenaline (NE) release during ripples in subcortical and cortical targets of the Locus Coeruleus (LC) may be beneficial for memory consolidation. Rats implanted with linear electrode arrays for extracellular recording in cortex and hippocampus and a stimulating electrode in LC were trained on a spatial memory task. Neural activity was monitored for 1h immediately after each learning session. Ripples were detected on-line using a band-pass filtered (150-250Hz) extracellular voltage signal recorded in the CA1 region of hippocampus by applying a threshold-crossing algorithm. Trains of biphasic electrical pulses (0.4ms, 0.05mA) were delivered to LC at each ripple onset. Group1 received LC stimulation (5 pulses at 20Hz) that did not produce detectable changes in cortical or hippocampal neural activity. Group2 received LC stimulation (10-20 pulses at 50-100Hz) that induced a transient (1-2s) desynchronization of cortical EEG, during which both thalamocortical sleep spindles and hippocampal ripples were suppressed. Additional control groups included random LC
stimulation, stimulation outside of LC, and sham-operated animals. Ripple-triggered LC stimulation produced a spatial memory deficit exclusively in Group2 rats, while behavioral performance of other control rats did not differ from intact animals. The stimulation-induced discharge of LC neurons and concurrent NE release caused a transient state change in the thalamocortical network, which was not favorable for hippocampal-cortical communication. These results challenge the original hypothesis, yet support the findings of our recent fMRI study showing a remarkable dichotomy between ripple-associated cortical activation and
deactivation of many subcortical regions including thalamus and brain stem neuromodulatory centers (Logothetis et al., 2012). CiteID: Eschenko2014_2

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Logothetis NK, Besserve M, Eschenko O, Murayama Y, Augath M, Steudel T, Evrard HC and Oeltermann A (July-2013) Keynote Lecture: Studying large-scale brain networks: electrical stimulation and neural-event-triggered fMRI, Twenty-Second Annual Computational Neuroscience Meeting (CNS*2013), Paris, France, BMC Neuroscience14 (Supplement 1) 1. The brain is "the" example of an adaptive, complex system. It is characterized by ultra-high structural complexity and massive connectivity, both of which change and evolve in response to experience. Information related to sensors and effectors is processed in both a parallel and a hierarchical fashion. The connectivity between different hierarchical levels is bidirectional, and its effectiveness is continuously controlled by specific associational and neuromodulatory centers. In the study of such systems one major problem is the adequate definition for an elementary operational unit (often called an "agent"), because any such module can be a complex system in its own right and may be recursively decomposed into other sets of units. A second difficulty arises from the synergistic organization of complex systems and of the brain in particular. Synergy here refers to the fact that the behavior of an integral, aggregate, whole system cannot be trivially reduced to, or predicted from, the components themselves. Localizing and comprehending the neural mechanisms underlying our cognitive capacities demands the combination of multimodal methodologies, i.e. it demands concurrent study of components and networks; one way of doing this, is to combine invasive methods which afford us direct access to the brain's electrical activity at the microcircuit level with global imaging technologies such as magnetic resonance imaging (MRI). In my talk, I'll discuss two such methodologies: Direct Electrical Stimulation and fMRI (DES-fMRI) and Neural-Event-Triggered fMRI (NET-fMRI).
DES-fMRI can be used in hopes of gaining insight into the functional or effective connectivity underlying DES-induced behaviors. Yet, our first findings suggest that DES has an important limitation: It clearly demarcates all monosynaptic targets of a stimulated site, but it largely fails to reveal polysynaptic cortico-cortical connectivity.
NET-fMRI, on the other hand, appears to offer great potential for mapping whole-brain activity that is associated with individual local events. In the second part of my talk, I'll describe the characteristic states of widespread cortical and subcortical networks that are associated with the occurrence of hippocampal sharp waves and ripples; the brief aperiodic episodes associated with memory consolidation.
CiteID: LogothetisEMASEBO2013

Eschenko O and Sara S (September-2009) Abstract Talk: Locus coeruleus activity during sleep for off-line memory consolidation, 41st European Brain and Behaviour Society Meeting, Rhodos, Greece, Frontiers in Behavioral Neuroscience Conference Abstract: 41st European Brain and Behaviour Society Meeting . Noradrenergic modulation has been hypothesized to contribute to memory consolidation by promoting synaptic plasticity in recently activated neural circuitries [1]. Behavioral studies identified a time window of ~ 2h after learning when noradrenergic influence on memory consolidation is most pronounced. Behavioral data are complemented with studies of long-term potentiation (LTP) or long-term depression (LTD), a cellular models of memory formation. One important outcome of these studies is that noradrenaline (NA) is required for late, protein-dependent phase of synaptic plasticity. According to the consolidation hypothesis, memory formation is a long-lasting process and thus continues after actual learning experience, i.e. off-line. Recently, sleep-mediated mechanisms of off-line information processing has drawn a lot of attention. A number of human and animal studies have convincingly shown the beneficial role of slow wave sleep (SWS) for memory consolidation. The activity of brain stem nucleus Locus Coeruleus (LC) - a major source of NA in the forebrain - is low, but not absent during SWS. By directly monitoring spiking activity of LC in behaving rats, we have recently revealed a transient surge of LC activity that occurred during SWS at around 2h after learning. In the present study we aimed to characterize LC activity in relation to SWS-associated cortical oscillations. Both slow waves (~ 1Hz) and spindles (~12Hz) are modulated by learning. LC activity is highly synchronized during SWS. Firing of LC neurons mostly occurs during the transition from cortical down to up state defined by a phase of slow oscillations. The LC activity is elevated immediately after the spindle onset. Taking into account experience-dependent replay of neuronal assemblies in multiple brain regions time-locking of LC activity to major cortical rhythms suggests synchronous noradrenergic modulation promoting synaptic plasticity in multiple and functionally connected brain sites. CiteID: EschenkoS2009

Safaai H, Neves R, Eschenko O, Logothetis NK and Panzeri S (October-2015) Modeling the effect of locus coeruleus firing on cortical state dynamics and single-trial sensory processingProceedings of the National Academy of Sciences of the United States of America112(41) 12834–12839.

Yeshenko O, Guazzelli A and Mizumori SJ (August-2004) Context-dependent reorganization of spatial and movement representations by simultaneously recorded hippocampal and striatal neurons during performance of allocentric and egocentric tasksBehavioral Neuroscience118(4) 751-769.

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Nikolskaya K and Echenko O (January-2002) Alcohol addiction as the result of cognitive activity in altered natural magnetic fieldElectromagnetic Biology and Medicine21(1) 1-18.